In regenerative air preheaters hot exhaust gases give up their heat to a mass of heat absorbent material that in turn gives up its heat to cool air or other gases flowing therethrough.
Instruments have been developed that are directed at the mass of heat absorbent material to detect the temperature thereof as a prerequisite for detecting incipient fires and initiating fire control within the air preheater. When viewing lenses are included in the temperature detection instruments to focus the rays upon a detector, the viewing lenses frequently become clouded or dirty because of the contaminated, corrosive atmosphere in which they must be immersed. As the viewing lenses become dirty they fail to transmit sufficient light so the detection instruments themselves fail to give a true indication of temperature or other conditions within the preheater.